Elevator Shaft Pressurization

Elevator Piston Effect

Elevator cars moving within shafts displace air volumes equal to car cross-sectional area times travel distance. This displacement creates transient pressure pulses—the piston effect—that superimpose on static stack effect pressures.

For an elevator car with:

Cross-sectional area: 50 ft² (typical 3000 lb capacity)
Speed: 1000 ft/min (high-rise elevator)
Shaft cross-sectional area: 100 ft²

The volumetric displacement rate equals:

$$Q_{displacement} = A_{car} \times v = 50 \times 1000 = 50,000 \text{ cfm}$$

This air must flow around the car, through door clearances, or through shaft openings. The resulting pressure pulse depends on shaft leakage characteristics and car speed.

Pressure Pulse Magnitude

Instantaneous pressure rise during car acceleration:

$$\Delta P_{piston} = \frac{\rho v^2}{2 g_c} \times \frac{(A_{car}/A_{shaft})^2}{1 - (A_{car}/A_{shaft})^2}$$

Where:

$\rho$ = air density (0.075 lbm/ft³ at standard conditions)
$v$ = car velocity (ft/min converted to ft/s)
$g_c$ = gravitational constant (32.2 lbm·ft/lbf·s²)
$A_{car}$ = car cross-sectional area (ft²)
$A_{shaft}$ = shaft clear area around car (ft²)

For the example elevator at full speed:

$$\Delta P_{piston} = \frac{0.075 \times (1000/60)^2}{2 \times 32.2} \times \frac{(50/100)^2}{1 - (50/100)^2} = 0.09 \text{ in. w.c.}$$

This pressure pulse adds to static stack effect pressure. Multiple elevators operating simultaneously create cumulative effects.

Operational Effects

Piston effect pressures affect:

Passenger Comfort: Rapid pressure changes cause ear discomfort (barotrauma). Pressure change rate exceeding 0.5 in. w.c./min creates noticeable effects. High-speed elevators require pressure equalization systems.

Door Operation: Transient pressures affect door opening and closing. Pressure assists or resists door operator mechanisms. Excessive pressure prevents proper door closure. Safety sensors may interpret pressure as obstruction.

Shaft Ventilation: Piston effect drives air circulation in shaft. Continuous elevator operation ventilates shaft naturally. Intermittent operation creates pressure fluctuations. Shaft temperature and air quality affected by circulation patterns.

Adjacent Spaces: Pressure pulses transmitted through shaft walls and doors. Ceiling tiles in elevator lobbies may lift during car passage. Noise transmission increases with pressure fluctuation magnitude. Sensitive spaces require acoustic and pressure isolation.

Door Opening Pressure Forces

Pressure differential across elevator doors creates resistance to opening. The force required to overcome pressure differential:

$$F_{pressure} = \Delta P \times A_{door}$$

Where:

$F_{pressure}$ = force due to pressure (lbf)
$\Delta P$ = pressure differential (lbf/ft²)
$A_{door}$ = door leaf area (ft²)

For standard elevator doors:

Height: 7 ft
Width (single leaf): 3.5 ft
Area per leaf: 24.5 ft²

At 0.30 in. w.c. pressure differential (0.0361 lbf/in² = 5.2 lbf/ft²):

$$F_{pressure} = 5.2 \times 24.5 = 127 \text{ lbf}$$

This force significantly exceeds door operator capacity (typically 50-75 lbf). Excessive pressure prevents door opening or causes operator failure.

Code Requirements

ASME A17.1 limits pressure effects on elevator doors:

Maximum pressure differential for normal operation: not specified directly
Door operator must function with anticipated pressure differentials
Door closing force limited to 135 lbf (kinetic energy method)
Pressure relief required to prevent excessive differentials

Practical design limits:

Target maximum: 0.15-0.20 in. w.c. for reliable door operation
Absolute maximum: 0.35 in. w.c. before door malfunction likely
Pressure relief activation: 0.25-0.30 in. w.c.

Measurement and Testing

Verifying door operation under pressure:

Pressure Monitoring: Install differential pressure sensors across shaft doors. Monitor during normal operation over full range of outdoor temperatures. Identify maximum pressure conditions (coldest winter day, highest stack effect).
Door Force Testing: Measure door opening force with calibrated force gauge. Test at floors experiencing maximum pressure differential (top and bottom floors). Verify forces within operator specifications.
Functional Testing: Operate doors repeatedly under maximum pressure conditions. Verify consistent performance over multiple cycles. Test both opening and closing operations. Confirm safety systems function properly under pressure.

Ventilation Requirements (ASME A17.1)

ASME A17.1 specifies elevator shaft ventilation to address:

Heat dissipation from machinery and car operation
Air quality in shaft environment
Smoke exhaust during fire emergencies (for designated fire service elevators)

Standard Ventilation

Natural Ventilation: Required for non-fire-service elevators. Minimum ventilation area: 3.5% of shaft cross-sectional area. Openings located at top of shaft (preferred) or distributed vertically. Openings protected from weather (louvers, hoods).

Mechanical Ventilation: Alternative to natural ventilation. Required when shaft lacks suitable locations for natural vents. Minimum air change rate: typically 1-2 ACH for temperature control. Continuous or intermittent operation based on heat load.

Ventilation Calculation: Heat dissipation in shaft includes:

Motor and machinery losses: 10-20% of motor nameplate power
Brake resistor heat (regenerative systems): variable, significant during deceleration
Heat transfer through shaft walls: typically minimal due to insulated construction
Solar gain through penthouse glazing: site-specific

Required ventilation rate:

$$Q_{vent} = \frac{q_{total}}{1.08 \times \Delta T}$$

Where:

$Q_{vent}$ = ventilation airflow (cfm)
$q_{total}$ = total heat dissipation (Btu/hr)
$\Delta T$ = allowable temperature rise (°F, typically 10-20°F)
1.08 = constant for standard air properties

Fire Service Elevator Ventilation

Fire service elevators require enhanced ventilation:

Smoke Exhaust Capability: Mechanical exhaust system in shaft. Minimum capacity: 4 complete air changes per hour. Activation upon fire alarm or manual command. Exhaust discharge to exterior, away from air intakes.

Pressurization Option: Alternative to exhaust: positive shaft pressurization. Supply air maintains shaft pressure above surrounding spaces. Prevents smoke infiltration during firefighter access. Typical pressure target: 0.10-0.15 in. w.c.

Machine Room Ventilation: Separate ventilation system for machine room. Maintains equipment within operating temperature limits. Independent from shaft ventilation. Mechanical ventilation required; natural ventilation typically inadequate.

Relief Venting Strategies

Pressure relief prevents excessive shaft pressurization from stack effect and piston effect.

Passive Relief Vents

Barometric Dampers: Spring-loaded or gravity-operated dampers opening at predetermined pressure. Typical opening pressure: 0.05-0.10 in. w.c. Location: shaft top or at intermediate floors. Advantages: no power required, simple operation. Disadvantages: fixed set point, potential for air leakage when closed.

Self-Closing Relief Vents: Normally closed louvers or dampers. Magnetic or mechanical latch holds closed. Pressure differential overcomes holding force to open. Automatic closure when pressure equalizes.

Sizing Relief Vents: Relief vent area must accommodate maximum airflow from piston effect and stack effect:

$$A_{relief} = \frac{Q_{max}}{V_{vent}}$$

Where:

$A_{relief}$ = required relief vent free area (ft²)
$Q_{max}$ = maximum airflow (cfm from piston effect calculation)
$V_{vent}$ = vent velocity (typically 500-1000 fpm to limit noise)

For 50,000 cfm piston displacement at 800 fpm velocity:

$$A_{relief} = \frac{50,000}{800} = 62.5 \text{ ft}^2$$

This represents free area after accounting for louver efficiency (typically 50-70%).

Active Relief Systems

Pressure-Controlled Exhaust: Motorized damper or dedicated exhaust fan. Differential pressure sensor triggers operation. Variable speed fan modulates to maintain target pressure. Integration with building automation system.

Relief Air Fan: Dedicated fan exhausting from shaft. Interlocked with elevator operation (operates when car in motion). Variable speed based on pressure differential. Discharge to building exhaust system or directly to exterior.

Advantages:

Precise pressure control
Adjustable set points for varying conditions
Integration with smoke control systems
Minimal air leakage when not operating

Disadvantages:

Requires electrical power and controls
Maintenance of mechanical components
Complexity compared to passive systems
Potential single point of failure

Relief Vent Placement

Location of relief vents affects effectiveness:

Top of Shaft: Most effective for winter stack effect (upward pressure). Simplest installation and weather protection. Standard for ASME A17.1 natural ventilation requirement. May be inadequate for summer stack effect (downward pressure).

Bottom of Shaft: Effective for summer stack effect. Problematic for moisture infiltration and contamination. Rarely used due to operational concerns. May be combined with top vents for year-round effectiveness.

Mid-Shaft: Reduces maximum pressure differential by providing relief at intermediate height. Requires penetration through occupied floors (aesthetic and functional challenges). May serve dual purpose as shaft ventilation and pressure relief.

Multiple Locations: Distributed vents at several shaft levels. Reduces maximum pressure differential throughout height. Increases installation and maintenance complexity. Most effective for super-tall buildings (> 600 feet).

Integration with Smoke Control

Elevator shafts interact with building smoke control systems:

Non-Fire-Service Elevators

During fire conditions:

Elevators recalled to designated landing (typically ground floor)
Doors remain closed during fire emergency
Shaft pressure prevents smoke infiltration if building under positive pressure
Stack effect may pull smoke into shaft if building under negative pressure
Relief vents may require fire dampers if penetrating fire-rated construction

Fire Service Elevators

Designated for firefighter access during emergencies:

Shaft Pressurization: Positive pressure relative to building spaces. Target: 0.10-0.15 in. w.c. minimum. Supply air via dedicated fan system. Maintains clean air in shaft for firefighter protection.

Lobby Pressurization: Elevator lobby on fire floor requires pressurization. Coordinated with shaft pressurization system. Prevents smoke infiltration while doors open. Typical target: 0.05-0.10 in. w.c. relative to fire floor.

Pressure Control During Door Opening: Opening shaft doors releases pressurized air. System must maintain minimum pressure with doors open. Requires higher supply air capacity than sealed shaft. Design for worst-case: multiple doors open simultaneously.

Control Strategy:

Fire alarm activation triggers pressurization system
Supply fan ramps to design airflow
Pressure sensors verify target pressure achieved
System modulates based on door position feedback
Backup power ensures operation during utility failure

Design Guidelines

Effective elevator shaft pressure management requires:

New Construction

Shaft Sealing: Specify tight construction tolerances. Continuous shaft walls with minimal penetrations. Gasketed shaft doors with automatic closers. Firestopping of all penetrations with tested assemblies.
Pressure Relief: Calculate maximum pressure from stack effect and piston effect. Size relief vents for maximum airflow. Locate vents for optimal performance. Specify automatic or barometric operation.
Ventilation Integration: Coordinate pressure relief with ASME A17.1 ventilation requirements. Single system serving both functions where possible. Separate systems for fire service elevator shafts.
Door Specifications: Select door operators rated for anticipated pressure differentials. Specify door closing force within code limits. Provide adjustable operating parameters for field optimization.

Existing Buildings

Pressure Measurement: Conduct baseline pressure surveys across range of outdoor temperatures. Identify floors and conditions with excessive pressure. Correlate pressure with door operation complaints.
Leakage Assessment: Quantify shaft leakage through blower door testing or tracer gas studies. Identify major leakage paths. Prioritize sealing efforts based on cost-effectiveness.
Relief Vent Addition: Install relief vents if pressure exceeds design limits. Size based on measured pressure and calculated stack effect. Coordinate with building structure and architectural constraints.
Control Optimization: Adjust building HVAC system operation to minimize shaft pressurization. Optimize supply/exhaust balance. Consider dedicated shaft pressure control systems for severe cases.

Elevator shaft pressurization represents a significant design challenge in tall buildings. The combination of stack effect and piston effect creates pressure conditions that affect door operation, passenger comfort, and life safety. Proper relief vent design, shaft sealing, and integration with building systems ensures reliable elevator operation under all environmental conditions.